1. Field of the Invention
This invention relates to a crystalline azopyrazolone acid dye having improved properties as compared to a similar azopyrazolone acid dye which is prepared by conventional prior art diazotization and coupling reactions.
2. Description of the Prior Art
Water-soluble azopyrazolone acid dyes are well known in the art and can be prepared by conventional diazotization and coupling reactions. Such dyes, after completion of the coupling reaction, carried out at a pH of, for example, 5-13, usually are isolated from the coupling reaction mix slurry by filtration, which step may be preceded by a salting out and/or acidification step. Representative of the prior art on the preparation of such dyes are U.S. Pat. Nos. 984,900; 3,563,685; and 3,575,957, British Pat. No. 966,677 and German Auslegeschrift No. 1,770,930. The German publication also discloses a method of improving the color stability of such a dye by heating the dye in an aqueous medium to about 50.degree. C., the heat-treated dye being defined by lattice plane separations of 6.17, 3.33, 18.50 and 3.52 A as indicated by the strongest lines in the Debye-Scherrer X-ray diagram. Azopyrazolone acid dyes prepared by prior art processes may be contaminated with inorganic impurities, for example, with at least 5-10% of the salt used in the salting out step, and they may be difficult to filter because of their small particle size, for example, of the order of five microns or less. Moreover, the presence of the salt as an impurity may retard the dissolution of the dye in a subsequent dyeing operation.